Without seeing the behavior it's awfully hard to say one way or the other. But, does it matter?1) is she playing?
IMO, no. It is not normal for dogs to forcibly knock other dogs down. If this happens with EVERY dog, clearly your dog may have some play skill deficiencies. If it happens randomly, your dog could very well be targeting particular dogs, and this is not play.2) is it normal?
Get help. You'll need someone qualified to decipher if it is just a play deficiency, resource guarding, or bullying. The what to do part is dependent on which it is. I don't know that you need a certified behaviorist, but I would start with a certified trainer who has access or can recommend a behaviorist should it be something outside of their practice.3) what can i do about it?